Chapter 4: Tissue—The Living Fabric (Anatomy & Physiology Study Notes)

The Living Fabric: Introduction to Human Tissues

Human body cells are highly specialized, with each type performing specific functions essential for maintaining homeostasis. Groups of similar cells that perform a common or related function are called tissues. There are four basic tissue types in the human body: epithelial, connective, muscle, and nervous tissue.

• Tissue: A group of cells similar in structure and function.

• Homeostasis: The maintenance of a stable internal environment.

• Main tissue types: Epithelial, connective, muscle, nervous.

Microscopy of Human Tissue

Preparation and Observation

To study tissues under a microscope, they must undergo specific preparation steps to preserve their structure and enhance visibility.

• Fixation: Tissue is preserved with a solvent to prevent decay.

• Sectioning: Tissue is cut into thin slices to allow light or electrons to pass through.

• Staining: Dyes or heavy metals are used to enhance contrast, though this can introduce artifacts (distortions).

• Light microscopy: Uses colored dyes.

• Electron microscopy: Uses heavy metal coatings for greater detail.

Epithelial Tissue

Definition and Functions

Epithelial tissue (epithelium) consists of sheets of cells that cover body surfaces or line cavities. It exists in two main forms:

• Covering and lining epithelia: Forms the outer layer of the skin and lines open cavities of the digestive and respiratory systems.

• Glandular epithelia: Forms the secretory tissue in glands (e.g., salivary glands).

Functions: Protection, absorption, filtration, excretion, secretion, and sensory reception.

Characteristics of Epithelial Tissue

Epithelial tissue is defined by five key characteristics:

• Polarity: Has an apical (top) surface and a basal (bottom) surface, each with distinct structures and functions.

• Specialized contacts: Cells are closely packed and joined by tight junctions and desmosomes to form continuous sheets.

• Supported by connective tissues: The basal surface is attached to a basal lamina, which is supported by a reticular lamina (part of the basement membrane).

• Avascular, but innervated: Contains no blood vessels but is supplied by nerve fibers; nutrients diffuse from underlying connective tissue.

• Regeneration: High capacity for renewal, especially in areas subject to friction or hostile environments.

Polarity in Epithelial Tissue

• Apical surface: Exposed to the body exterior or cavity; may have microvilli or cilia.

• Basal surface: Attached to underlying connective tissue via the basal lamina.

Basement membrane: Composed of the basal and reticular laminae, reinforces the epithelium and resists stretching and tearing.

Classification of Epithelia

Epithelia are classified by the number of cell layers and the shape of the cells:

• Number of layers:

  • Simple epithelia: Single layer of cells (for absorption, secretion, filtration).

  • Stratified epithelia: Two or more layers (for protection).

• Cell shape:

  • Squamous: Flattened and scale-like.

  • Cuboidal: Box-like, as tall as they are wide.

  • Columnar: Tall and column-shaped.

In stratified epithelia, cell shape is named according to the shape of the apical layer.

Types of Epithelia and Their Functions

• Simple squamous epithelium: Thin and permeable; found where rapid diffusion is required (e.g., alveoli of lungs, lining of blood vessels).

• Simple cuboidal epithelium: Secretion and absorption; found in kidney tubules and small glands.

• Simple columnar epithelium: Absorption and secretion; lines most of the digestive tract.

• Pseudostratified columnar epithelium: Appears stratified but is a single layer; found in respiratory tract.

• Stratified squamous epithelium: Protects underlying tissues; found in skin, mouth, and esophagus.

• Stratified cuboidal and columnar epithelium: Rare; found in some ducts (e.g., sweat glands, male urethra).

• Transitional epithelium: Stretches to permit distension; lines urinary bladder and ureters.

Glandular Epithelia

A gland consists of one or more cells that make and secrete a particular product (secretion).

• Endocrine glands: Ductless; secrete hormones directly into the blood or lymph.

• Exocrine glands: Secrete products into ducts that open onto body surfaces (e.g., sweat, oil, salivary glands).

• Unicellular exocrine glands: Single cells (e.g., goblet cells) that produce mucin.

• Multicellular exocrine glands: Composed of a duct and a secretory unit; classified by duct structure (simple or compound) and secretory unit shape (tubular, alveolar, or tubuloalveolar).

Modes of Secretion

• Merocrine: Secrete products by exocytosis (e.g., sweat glands, pancreas).

• Holocrine: Accumulate products until the cell ruptures (e.g., sebaceous glands).

• Apocrine: Accumulate products at the apex, which pinches off (controversial in humans; possible in mammary glands).

Connective Tissue

Overview and Functions

Connective tissue is the most abundant and widely distributed tissue type. It provides binding and support, protection, insulation, energy storage, and transportation of substances (blood).

• Main classes: Connective tissue proper, cartilage, bone, blood.

Structural Elements of Connective Tissue

• Ground substance: Gel-like material that fills the space between cells; contains interstitial fluid, adhesion proteins, and proteoglycans.

• Fibers: Provide support; three types:

  • Collagen fibers: Strongest and most abundant; high tensile strength.

  • Elastic fibers: Long, thin, and allow for stretch and recoil.

  • Reticular fibers: Short, fine, and form networks for support.

• Cells: Each major class has a resident cell type:

  • "Blast" cells: Immature, actively mitotic cells that secrete matrix (e.g., fibroblasts in connective tissue proper, chondroblasts in cartilage, osteoblasts in bone).

  • "Cyte" cells: Mature, less active cells that maintain the matrix.

  • Other cells: Fat cells (store nutrients), white blood cells (tissue response to injury), mast cells (inflammation), macrophages (phagocytosis).

Types of Connective Tissue

• Connective tissue proper: Includes loose (areolar, adipose, reticular) and dense (regular, irregular, elastic) types.

• Cartilage: Hyaline, elastic, and fibrocartilage; avascular and flexible.

• Bone (osseous tissue): Supports and protects; stores fat and synthesizes blood cells; highly vascularized.

• Blood: Atypical connective tissue; consists of cells in a fluid matrix (plasma); transports nutrients, wastes, and gases.

Comparison of Connective Tissue Classes

Type	Matrix	Fibers	Cell Types	Vascularity
Connective tissue proper	Gel-like	All three (collagen, elastic, reticular)	Fibroblasts, fibrocytes, defense cells, fat cells	Varies (well vascularized)
Cartilage	Gel-like	Collagen, elastic (some types)	Chondroblasts, chondrocytes	Avascular
Bone	Hard, calcified	Collagen	Osteoblasts, osteocytes	Highly vascularized
Blood	Fluid (plasma)	None (soluble proteins)	Red and white blood cells, platelets	Present

Muscle Tissue

Muscle tissue is responsible for most types of body movement. Muscle cells contain myofilaments (actin and myosin) that contract to produce movement.

• Skeletal muscle: Voluntary movement; attached to bones.

• Cardiac muscle: Involuntary; found only in the heart.

• Smooth muscle: Involuntary; found in walls of hollow organs (e.g., intestines, blood vessels).

Nervous Tissue

Nervous tissue is specialized for communication by electrical and chemical signals. It consists of neurons (nerve cells) and supporting cells (neuroglia).

• Neurons: Transmit electrical impulses.

• Neuroglia: Support, insulate, and protect neurons.

Covering and Lining Membranes

Body membranes are composed of an epithelial layer and an underlying connective tissue layer. The main types are:

• Cutaneous membrane: The skin; composed of stratified squamous epithelium and dense connective tissue.

• Mucous membranes (mucosae): Line body cavities open to the exterior (e.g., digestive, respiratory tracts); secrete mucus.

• Serous membranes (serosae): Line closed ventral body cavities and cover organs; secrete serous fluid for lubrication.

Tissue Repair

When tissues are injured, the body initiates an inflammatory response and repair process. Repair can occur in two ways:

• Regeneration: Replacement of destroyed tissue with the same kind of tissue; restores function.

• Fibrosis: Replacement with scar tissue (connective tissue); function is lost.

The capacity for regeneration varies by tissue type:

• High regenerative capacity: Epithelial tissues, bone, areolar connective tissue, dense irregular connective tissue, blood-forming tissue.

• Moderate: Smooth muscle, dense regular connective tissue.

• Low/none: Cardiac muscle, nervous tissue in brain and spinal cord.

Developmental Aspects of Tissues

All tissues originate from three primary germ layers formed early in embryonic development:

• Ectoderm: Gives rise to nervous tissue and epidermis.

• Mesoderm: Gives rise to muscle and connective tissues.

• Endoderm: Forms the lining of the digestive and respiratory systems.

With aging, tissue repair becomes less efficient, epithelia thin, and the risk of cancer increases.